Field of the Invention
The invention relates broadly to the field of telecommunications and particularly to wireless telecommunication systems. More particularly, it relates to a method for efficiently and flexibly transmitting both dedicated and simulcast services over wireless systems by judicious allocation and use of physical communication channels. The technology particularly suitable for providing these services is a combination of Time Division Multiple Access (TDMA) and Orthogonal Frequency Division Multiplexing (OFDM).
Description of the Related Art
Existing wireless systems are configured either for two-way dedicated services or for broadcasting services. The two types of services use substantially different technologies and are not amenable to simple, inexpensive and efficient common network implementations.
Two-way services are dedicated communications between two stations, for example, between a base station and a single mobile subscriber station. Two-way services between two mobile subscribers must, by definition, pass through a base station so, for purposes of the present discussion, we consider only the transmission between the base station and a single mobile subscriber station, whether that communication is in one direction or two directions. Two-way services may also include, for example, interactive communication between a mobile subscriber and an Internet Web site. For purposes of the present invention, two-way communications are a subset of dedicated communications, which can be one-way or two-way but are always addressed to a single user. Current wireless mobile systems are designed to carry primarily two-way dedicated services.
Wireless broadcasting systems use fixed and inflexible channels for transmitting a single one-directional stream of information from one transmitter to numerous terminals, as opposed to the use of wireless systems. Wireless systems contain common control channels, which are operated in the broadcast mode, but this type of broadcast channel is configured to provide a minimal amount of information and is used primarily as a support for dedicated information channels. Short message service (SMS) can be carried over broadcast channels, but there are significant limitations in the length of SMS driven by physical constraints of technologies used for current generation wireless systems.
Apart from broadcast and dedicated transmission modes, certain applications can take advantage of the simulcast transmission mode. In the simulcast mode, a multiplicity of base stations transmits the same information to one or more terminals. Simulcast can be viewed as an aggregation of multiple simultaneous limited-area broadcasts from a number of base stations, or as an efficient replacement for a large area broadcast implemented within the wireless network. Simulcasting can be used for paging and high information rate distribution as well as for audio/video, e.g., digital broadcasting systems. An example of simulcast paging might be to alert volunteer emergency services personnel (e.g., volunteer firefighters) of the need to respond, rather than depending solely on the old siren. Another example of simulcasting may be an efficient distribution of stock market news by minimizing wireless resources of the wireless system.
Both current Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA) wireless systems have broadcast control channels that have severe limitations on their use for information transmission. Traditional TDMA systems that could be redesigned based on traditional technologies to provide high information rate simulcast transmission will not be efficient. While in the soft-handoff, CDMA systems operate in a special version of the simulcast mode, where information is sent from many base stations to a single terminal, rather than information being sent from a base station (or base stations) to a multiplicity of terminals. However, providing simulcast to mobile stations outside of the soft-handoff region is not feasible with CDMA. As opposed to TDMA and CDMA technologies, the combination of TDMA with Orthogonal Frequency Division Multiplexing (OFDM), which we describe in this invention, provides a basis for highly efficient simulcasting which is very well integrated with high-speed dedicated services in a common wireless system network.